Several different types of chromogenic lactone compounds are described in U.S. patent reissue No. 23,024, U.S. Pat. Nos. 3,491,112, 3,491,116, 3,509,173, 3,540,909, 3,540,911, 3,540,912, 3,736,337, 3,775,424, 3,853,869 and 4,020,068 and in Belgian patent No. 844,962.
U.S. Pat. No. 3,958,815 describes carbinol ether compounds containing indolizinyl groups.